A number of electrical-output temperature-measuring instruments have been provided heretofore, based on various principles whereby temperature is associated with variations in electrical parameters. For example, thermocouples, consisting of a junction of two different metals, are provided to generate a voltage proportional to temperature and thereby allow measurement of the electrical output to be calibrated directly in temperature terms. In another type of temperature sensor, the fact that a thermal coefficient of electrical resistance exists is used to permit a sensitive element to be connected in a resistance bridge whose output represents the temperature. Other electrical parameters of semi-conductors and conductors have also been used in the detection of temperature changes and in the measurement of temperatures.
The present invention deals with temperature measurement using the noise voltage produced in a conductor. It has been determined by others that noise voltage or thermal noise (also known as Johnson noise) can be produced in a conductor by thermal agitation of electrically-charged particles within the conductor. The available thermal noise power is proportional to the resistance value and is proportional to the absolute temperature and, of course the frequency bandwidth over which the noise is measured. The electrical output is proportional to absolute temperature as indicated.
It is known that all metals have the noise-generation properties mentioned above, i.e. produce a noise voltage which is proportional to temperature. For the most part, platinum has been used heretofore in temperature sensors operated under the noise-voltage principle.
It should be observed that temperature sensors having a measurable noise-voltage output proportional to temperature have the advantage that the output is easily discriminated from changes and is not falsified by any such changes which may not be strictly proportional to temperature. Hence various disadvantages of resistance-temperature sensors can be avoided by the use of noise-voltage sensors. Another problem with conventional sensors has been the generally low output available for a given spatial arrangement of the sensing element.